It depends on the court. For example, at the appellate level in the United States Federal Courts, you would need to file a motion, not a brief.
The motion would have to request permission from the court to file the brief. A proposed brief would have to be filed with the motion, but would not be the same document.
(a) When Permitted. The United States or its officer or agency or a
state may file an amicus-curiae brief without the consent of the
parties or leave of court. Any other amicus curiae may file a brief
only by leave of court or if the brief states that all parties have
consented to its filing.
(b) Motion for Leave to File. The motion must be accompanied by the
proposed brief and state:
(1) the movant's interest; and
(2) the reason why an amicus brief is desirable and why the matters
asserted are relevant to the disposition of the case.